In AGT systems of the type mentioned above, it is a requirement that vehicles be capable of frequent stopping at stations located off the main line in order that a high mean track speed on the main line is achieved, notwithstanding that such stations may be spaced at close intervals.
It must be possible for alternate vehicles to follow either the mainline or turn off to a branch line, with a minimal loss of speed. As such, the switch length must be short and the switch actuation time minimal, for example, two seconds or less. Preferably any turning section which is used to branch from a main line should be banked for passenger comfort.
Conventional railway switches are not used for AGT systems for several reasons. First, AGT systems often use elevated guideways to avoid interference with ground-level traffic, and are too obtrusive in city or suburban environments unless the track is narrow, typically about one half the width of the vehicle, and are often termed monorail systems. In such systems, provision must be made by way of extra wheels and guide surfaces to prevent vehicles from overturning, for example, in high winds. Such extra guide surfaces are usually provided at a level underneath a flat, broad load carrying track so further complicating the design of switches and precluding the use of conventional railway switches.
Second, it is not practical to bank the turning rails in conventional railway switches.
Third, in AGT systems it is typical for power to be supplied from the guideway, for example, by longitudinal conductors as in the case of some conventional electric railways which use a third rail for this purpose; as overhead conductor wires are generally unsuited to elevated guideways on aesthetic grounds. In the case of AGTs, additional longitudinal conductors are also typically required to provide control and communication channels. Such groups of longitudinal conductors cannot intersect the running surfaces, and hence in conventional railway switches, both the conductors and the collecting brushes on the vehicle are duplicated on each side of the vehicle and track in order to provide a continuous electrical connection in the switch zone.
Some prior art proposals have attempted to overcome the problems associated with switching of monorail and other guideway systems by laterally shifting a first section of the guideway together with the longitudinal conductors and supplemental guiding surfaces, and moving into its place a second section, the first section of guideway being straight and flat and the second section curved and banked. However, the problem with such prior art proposals is that the switching time is long because of the need to move massive structures. Other prior art proposals have included the bending of an entire monorail structure including the switching zone, these proposals being generally related to fast trains where switching time is not important.
One impractical prior art proposal is that by Trent in U.S. Pat. Nos. 3,472,176 and 3,477,389, which show a narrow beam carrying a track surface which is bent and twisted by virtue of being supported by a series of vertical supports or posts. These posts are inclinable, being hinged at a point well below the surface of the ground, and arranged to provide for banking as well as bending of the track in the switch zone. The impracticality of such a proposal is that, first the switching structure may well extend for hundreds of meters, and with the beam dimensions disclosed could weigh many hundreds of tons. Second, it is not acceptable to relate the amount of side shift to the roll angle (or incline) which inevitably occurs within this arrangement. For example, the last pivoted support would extend about 23 meters into the ground if the recommended bank angle of fifteen degrees continued to that point. Additionally, as it is well known in the flexing behaviour of beams, the curvature decreases to zero for lateral loads applied at the end of a beam, so that the appropriate incline angle would also be zero, which Trents' structure could not provide.
Another impractical proposal is by J. Rosenbaum et. al. in U.S. Pat. Nos. 2,997,004 and 3,093,090, which disclose the use of a box type beam of narrow width (which is straddled by the railway carriages to provide stability) and which is bent sideways to provide side-shifting of the track to effect switching. However, as in the case of the earlier mentioned Trent proposal, this arrangement involves the moving laterally of the entire beam structure from one position to the other, which would be quite difficult. Neither Rosenbaum or Trent make provision for flexing the beam structure in twist which is desirable to provide banking.
Similarly G. Schutze in U.S. Pat. Nos. 3,013,504 and 2,903,972 shows a box type guideway having the same limitations as noted above.
The prior art switching proposals are not suited for use in AGT systems where individual vehicles travel in a stream at high speeds and at relatively close spacing from one another, and it is therefore an advantage if the vehicles are able to detour or switch from the main line or track when stopping at stations to allow through vehicles to pass without slowing down. Any switch in such an arrangement must be able to move from the main line to the branch line rapidly, and the switch length should preferably be short as mentioned earlier. This would result in passengers experiencing high lateral accelerations when the vehicle is traversing the curved track in the branch line position which would be more acceptable to passengers if the track is appropriately banked when in the branch position. If the track is narrow, and hence the required displacement is less, the degree of side acceleration is also less. Such a narrow track is described in co-pending International application PCT/AU94/00046 corresponding to co-pending U.S. patent application Ser. No. 08/500,862 entitled Self Steering Railway Bogie in which the guideway may be elevated and the vehicle is locked onto the track, for example by grip wheels as described in further co-pending international application PCT/AU94/00201 corresponding to co-pending U.S. patent application Ser. No. 08/537,792 entitled Rail Gripping Vehicle. Such a narrow track arrangement, in which grip wheels run on the underside of the rail head precludes the use of existing railway switching techniques and would make rail crossover areas impractical which is recognised in the earlier mentioned art of Rosenbaum, Trent and Schutze.